Additional background for therapeutic use of peptide fragments in the treatment of Alzheimer's disease and other amyloidoses can be found in U.S. patent application Ser. No. 09/938,275 filed Aug. 22, 2001, and in U.S. patent application Ser. No. 09/962,955 filed Sep. 24, 2001, the text and drawings of each of which are hereby incorporated by reference into the present application as if fully set forth herein.
Beta-Amyloid Protein as a Therapeutic Target for Alzheimer's Disease
Alzheimer's disease AD) is characterized by the deposition and accumulation of a 39-43 amino acid peptide termed the beta-amyloid protein, Aβ or β/A4 Glenner and Wong, Biochem. Biophys. Res. Comm. 120:885-890. 1984; Masters et al, Proc. Nat. Acad. Sci. U.S.A. 82:4245-4249, 1985; Husby et al, Bull. WHO 71:105-108, 1993). Aβ is derived from larger precursor proteins termed beta amyloid precursor proteins or APPs) of which there are several alternatively spliced variants. The most abundant forms of the APPs include proteins consisting of 695, 751 and 770 amino acids Kitaguchi et al, Nature 331:530-532, 1988; Ponte et al, Nature 331:525-527, 1988; Tanzi et al, Nature 331:528-530, 1988). The small Aβ peptide is a major component that makes up the core of amyloid deposits called “plaques” in the brains of patients with AD. In addition, AD is characterized by the presence of numerous neurofibrillary “tangles”, consisting of paired helical filaments which abnormally accumulate in the neuronal cytoplasm Grundke-lqbal et al Proc. Natl. Acad. Sci. U.S.A. 83:4913-4917, 1986; Kosik et al, Proc. Natl. Acad. Sci. U.S.A. 83:4044-4048, 1986; Lee et al, Science 251:675-678, 1991). The other major type of lesion found in AD brain is the accumulation of amyloid in the walls of blood vessels, both within the brain parenchyma and meningeal vessels that lie outside the brain. The amyloid deposits localized to the walls of blood vessels are referred to as cerebrovascular amyloid or congophilic angiopathy Mandybur, J. Neuropath. Exp. Neurol. 45:79-90, 1986; Pardridge et al, J. Neurochem. 49:1394-1401, 1987). The pathological hallmarks of AD therefore are the presence of “plaques”, “tangles”, and cerebrovascular amyloid deposits.
For many years there has been an ongoing scientific debate as to the importance of “amyloid” in AD and whether the “plaques” and “tangles” characteristic of this disease, were a cause or merely the consequences of the disease. Recent studies indicate that amyloid is indeed a causative factor for AD and should not be regarded merely as a consequence. The Alzheimer's Aβ protein in cell culture has been shown to cause degeneration of nerve cells within a short time period Pike et al, Br. Res. 563:311-314, 1991; J. Neurochem. 64:253-265, 1995). Studies suggest that it is the fibrillar structure, characteristic of all amyloids, that is mainly responsible for the neurologic effects. Aβ has also been found to be neurologic in slice cultures of hippocampus Hadrian et al, Neurobiol. Aging 16:779-789, 1995) and induces nerve cell death in transgenic mice Games et al, Nature 373:523-527, 1995; Hsiao et al, Science 274:99-102, 1996). Injection of Aβ into rat brain also causes memory impairment and neuronal dysfunction Flood et al, Proc. Natl. Acad. Sci. U.S.A. 88:3363-3366, 1991; Br. Res. 663:271-276, 1994). Convincing evidence that Aβ amyloid is directly involved in the pathogenesis of AD comes from genetic studies. It was discovered that the increased production of Aβ could result from mutations in the gene encoding, its precursor, APP Van Broeckhoven et al, Science 248:1120-1122, 1990; Murrell et al, Science 254:97-99, 1991; Haass et al, Nature Med. 1:1291-1296, 1995). The identification of mutations in the APP gene which causes early onset familial AD is a strong argument that Aβ and amyloid are central to the pathogenetic process underlying this disease. Four reported disease-causing mutations have now been discovered which demonstrate the importance of Aβ in causing familial AD reviewed in Hardy, Nature Gen. 1:233-234, 1992). Lastly, recent studies suggest that a reduction in amyloid plaque load in APP transgenic mice lead to improvements in behavioral impairment and memory loss Chen et al, Nature 408:978-982, 2000; Janus et al, Nature 408:979-982, 2000; Morgan et al, Nature 408:982-985, 2000). This is the strongest argument to date that implicates that reduction of Aβ amyloid load in brain should be a central target for the development of new and effective treatments of AD and related disorders.
In addition, besides Alzheimer's disease, a number of other beta-amyloid protein diseases involve formation, deposition, accumulation and persistence of Aβ fibrils, including Down's syndrome, disorders involving congophilic angiopathy, such as but not limited to, hereditary cerebral hemorrhage of the Dutch type, inclusion body myositosis, dementia pugilistica, cerebral β-amyloid angiopathy, dementia associated with progressive supranuclear palsy, dementia associated with cortical basal degeneration and mild cognitive impairment.
Modulators of APP Secretases as Therapeutic Targets for Alzheimer's Disease
Elucidating APP metabolism and its role in the formation of Aβ plaques in AD is becoming increasingly important as therapeutics for AD and other beta-amyloid protein diseases are sought. Intracellular trafficking and proteolytic processing of APP directly influences the amount and type of Aβ peptide and can thus have a profound impact on amyloid plaque load.
Processing of APP in vivo and in cultured cells occurs by two major pathways Haass and De Strooper, Science 2865441):916-9 1999) and; Selkoe, Physiol Rev. 81 2):741-66, 2001)). Cleavage of APP at the N-terminus of the Aβ region by β-secretase and at the C-terminus by γ-secretases represents the amyloidogenic pathway for processing of APP. β-secretase cleaves APP between residues Met595 and Asp596 codon numbering refers to the APP695 isoform), and yields Aβ peptide plus the β-C-terminal fragment βCTF or C99). Following β-secretase cleavage, a second cleavage by γ-secretase occurs at the C-terminus of Aβ peptide that releases Aβ from CTF. This cleavage occurs in the vicinity of residue 636 of the C-terminus. γ-secretase can cleave the C-terminal region at either Val636 or Ile638 to produce a shorter Aβ peptide Aβ1-40) or the longer Aβ peptide Aβ1-42). The predominant form of Aβ found in the cerebrospinal fluid and conditioned media of cultured cells is the shorter Aβ40 peptide. Despite its lower abundance, Aβ42 is the peptide that is initially deposited within the extracellular plaques of AD patients. In addition, Aβ42 is shown to aggregate at a much lower concentration than the Aβ40 form. Alternatively, APP can also be processed via the non-amyloidogenic pathway whereby α-secretase cleaves within the Aβ domain between Lys611 and Leu612, and produces a large soluble APP domain sAPPα) and the αC-terminal fragment αCTF or C83). The latter can then be cleaved by γ-secretase at residue 636 or 638 to release a P3 peptide and the APP intracellular domain AICD). The α-cleavage pathway is the major pathway used to process APP in vivo; it does not yield Aβ peptide Selkoe, Physiol Rev. 812):741-66, 2001). The characterization of APP cleavage and the related secretases has provided significant advancement in therapeutic strategies that may lead to limiting the deposition of Aβ peptide in the brain and eliminate or delay the associated pathological effects in AD.
Alzheimer's Disease and the Aging Population
Alzheimer's disease is a leading cause of dementia in the elderly, affecting 5-10% of the population over the age of 65 years Jorm, A Guide to Understanding of Alzheimer's Disease and Related Disorders, New York University Press, New York, 1987). In AD, the parts of the brain essential for cognitive processes such as memory, attention, language, and reasoning degenerate. In some inherited forms of AD, onset is in middle age, but more commonly, symptoms appear from the mid-60's onward. AD today affects 4-5 million Americans, with slightly more than half of these people receiving care in many different health care institutions. The prevalence of AD and other dementias doubles every 5 years beyond the age of 65, and recent studies indicate that nearly 50% of all people age 85 and older have symptoms of AD NIH Progress Report on AD, National Institute on Aging, 2000). Thirty-three million people of the total population of the United States are age 65 and older, and this will climb to 51 million people by the year 2025 NIH Progress Report on AD, National Institute on Aging, 2000). The annual economic toll of AD in the United States in terms of health care expenses and lost wages of both patients and their caregivers is estimated at $80 to $100 billion NIH Progress Report on AD, National Institute on Aging, 2000).